Impedance transformer



June 19, 1934.. J STERBA 1,963,723

IMPEDANCE TRANSFORMER Original Filed July 30, 1929 uvwsw ran E. J STERBA ATTORNEY Patented June 19, 1934 1,963,723 EVIPEDANCE TRANSFORMERErnest J. Sterba, Asbury Park,

Bell Telephone Laboratories,

N. J., assignor to Incorporated,

New York, N. Y., a corporation of New York Original application July 30,1929, Serial No.

382,103. Divided and this application September 16, 1930, Serial No.482,210

6 Claims.

This invention relates to transmission systems and particularly to radiofrequency transmission systems. This application is a division of myapplication which issued as Patent 1,885,151, U November 1, 1932.

As is well known, considerable energy loss results in the transmissionof high frequency energy over transmission conductors which areopen-ended or terminated by impedances having a value other than that ofthe surge impedance of the line. A part of this loss is due to the factthat the standing waves which are produced on the open-ended line byreflection radiate considerable energy. Heretofore, in order to obtainthe proper surge impedance for the line, coil type impedancetransformers or autotransformers have been employed which, aside frombeing inaccurate, are comparatively costly.

It is one object of this invention to eliminate standing waves ontransmission lines without the use of the conventional coil type surgeimpedance transformers.

It is another object of this invention to change the value of a load orterminating impedance, such as an antenna, to one equivalent to thesurge impedance of the associated line in a simple and inexpensivemanner. v

According to one feature of this invention, a transmission line isconnected to an antenna by v 30 means of a quarter wave length linewhose surge impedance equals the square'root of the product of the loadand sending impedances of the line. The load impedance is therebytransformed into the proper impedance for terminating the line and as aresult standing waves are eliminated from the line.

The invention will be more fully understood from the followingdescription taken in connection with the drawing in which:

Fig. 1 represents a simple embodiment of the invention;

Fig. 2 a directional transmitting system and associated transmissionline in which an impedance transformer is employed in accordance withthe invention.

Referring to Fig. 1 an impedance transformer comprising two conductors1, preferably made of copper tubing, is illustrated. The length of theseconductors is a quarter wave length and their spacing is such that'theresulting surge imped ance is suitable for transforming the loadimpedance 2 into the surge impedanceof line 3. The surge impedance ofline 3 is represented on the drawing by dotted resistance 4. Withrespect to the line comprising conductors 1, the

impedance of the line 3 is the sending impedance and impedance 2 is theload impedance. The surge impedance of the transformer comprising thetwo quarter wave length conductors may be determined mathematically asfollows:

The sending end impedance Z1 of any transmission line such as the linecomprising conductors 1 having a known terminating impedance such asimpedance 2 is given on page 99 of Flemings book entitled (ThePropagation of Electric, Currents in Telephone and Telegraph Conductors3rd Edition, as Equation (61), which is:

Z Z, cosh PI+Z sinh P1 1 Z cosh PI-l-Z, sinh P1 where Z1=sendingimpedance Zo=surge impedance of the line Zr=terminating impedance of theline P=propagation constant per u 't length Z=length of the line Ifthere is no loss in the line the propagation constant is a pureimaginary, and

letting the wave length and B: the total phase shift then Pl=aB, whereB:

z,+z0 coth P1 (2) Z1: (v3) Zr-1 0 Assuming the length of the line issuch that v n)\ and that n is any odd number, such as 1", then, inEquation (3) and, substituting in Equation (3) we have ita Z12. If n isany even number, as for example, 2, then, in Equation (3) And,substituting in, and separating the terms of Equation (3) we have,

for terminating a transmission line, the actual impedance Zr inwhich theline is terminated may be transformed into this ideal impedance byconmating the impedance Zr to a line a quarterwave length oran oddmultiple thereof long and whose surge impedance is designed to have avalue of Z0 which satisfies Equation (4). Consequently, lines an oddmultiple of a quarter wave length long may be employed as step-up orstep-down impedance transformers, in place of the usual coiltransformers or other similar cumbersome apparatus. 1

Referring again to Fig. 1, the impedance at point J will be the same asimpedance 2 since point J is an even multiple of a quarter wave lengthfrom impedance 2 along the transmission line. This impedance istransformed by means of the quarter wave line transformer comprisingconductors 1 into an impedance at point K, which, because of the halfwave length separation, has a value equal to that'of impedance 4 and is,by assumption, the proper impedance for terminating the transmissionline 3.

In Fig. 2 a perspective view of a complete unidirectional radiating orabsorbing array such as disclosed in my Patent 1,885,151 mentionedabove, isv shown, together with an associated transmission system inwhich one embodiment of the invention is utilized. The active antenna orexciter 5 consists of three panels arranged in a row and in the samevertical planeeach panel comprising two vertical sections spaced adistance equal to one-half of the wave length for which the system isdesigned. For convenience, only three panels are shown but of course itshould be understood that any number of panels within a certain limitmay be employed. Each vertical section comprises four half waveradiating elements, the top and bottom elements being'folded at themid-point toward the center of the panel in such a manner that theoutermost half of each top and bottom element is horizontal andtherefore easily joined, as shown in the drawing; to the connectalternate radiating elements of one vertical to the radiating elementsof the adjacent vertical corresponding in position to the elementsintermediate to said alternate elements.

An inactive antenna or reflector 6 also consisting of three panels issymmetrically located a quarter of a wave length to the rear of andparallel to the exciter; but unlike the exciter the panels of thereflector are not conductively associated with the transmission system.

The panel of the exciter and reflector are supported and kept inposition in the usual manner by means of elevated support 7 extendingbetween towers 8 and 9 and by means of grounded guy wires 10. Each panelis insulated from the ground wires and the elevated supports by means ofinsulators 11. The colinear radiating elements of each section and thehorizontal elements on the same level are interconnected by suitable guywires properly insulated, which for the sake of clearness, have beenomitted from the drawing.

Directly associated with the exciter is a transmission system in whichthe impedance transformer is utilized. The system comprises a main trunkline 12 connected to a source of radio frequency energy which is notshown on the drawing and to three branch lines 13 at junction point A,one branch line 13 being associated with each panel. The referencenumeral 14 refers to an impedance or line transformer which is insertedat junction A. The length of each branch line 13 is equal to an evenmultiple of one quarter of the wave length transmitted; and the lengthof the conductors comprising transformer 14 equals an odd multiple of a.quarter wave length. The transmission system is illustrated as lying inthe same horizontal but this is not a necessary condition and atconvenient points the line may extend in a vertical directio In theoperation of the system shown in Fig.2 energy supplied from the powersource over line 12 is transmitted by virtue of impedance transformer 14to the individual panel of the exciter with substantially no reflectionloss. As seen from junction A the terminating impedance is equivalent tothe three panel impedances in parallel since each of the threebranchlines is an even multiple of a quarter wave length long. Thisterminating or load impedance is transformed by means of transformer 14into an impedance equivalent tothe surge impedance of the main line 12.v

The energy supplied to the exciter 5 is radiated in the manner alreadyexplained in my copending application mentioned above. Energy ispropvertical, elements only, the radiation from the horizontalelementsbeing neutralized. The system is uni-lateral and because of the numberof verticals a concentrated directional effect is obtained in a lowlying angle of fire in the direction represented by arrow M.

Although the invention has been illustrated and described in connectionwith a particular type of radio system it is to be understood that it isapplicable to any high frequency aerial or transmission system and thatit is not to be limited to the specific embodiment described.

,What is claimed is:

1. The methodof preventing wave reflection on a transmission lineconnected to a load impedance utilizing another transmission line aquarter wave its surge impedance divided by the load impedance equalsthe surge impedance of the first mentioned line and inserting the'secondmentioned line between the first mentionedline and the load impedance.2. In a high frequency transmission system, a

first line a quarter wave length long or an odd multiple thereof, aterminating impedance connected to one terminal of said line, a secondline connected to the opposite terminal of said first line, the squareof the surge impedance of the first line being equal to the product ofthe terminating impedance and the surge impedance of the second line.

3. In a high frequency transmission system, a line comprising aplurality of conductors each having a length equal to one quarter of thewave length for which the system is designed or an odd multiple thereof,a load impedance connected to the said line, a source of energy, a mainline connected between said quarter wavelength line and the source, theload impedance and the surge impedance of said main line havingdifferent values, the spacing of the conductors being arranged toprevent reflection of waves into said source.

4. In a high frequency transmission system, a transformer comprising apair of conductors each having a length equal to a quarter wave lengthor an odd multiple thereof, an impedance, a source of energy, theimpedance being connected to one end of the transformer, a transmissionline connecting said source to the other end of the transformer, saidtransformer having a surge impedance equal to the square root of theprodtransmission lines connecting said antenna and translation system toopposite terminals of said transformer, the square of the surgeimpedance of the transformer divided by the antenna impedance beingequal to the surge impedance of the transmission line connecting saidtransformer and translation system.

6. In a high frequency system, a plurality of radiating elements, animpedance transformer comprising a plurality of lines a quarter wavelength long or an odd multiple thereof, a transmitter, a transmissionline connecting each of said elements to said transformer, atransmission line connecting said transmitter to said transformer, theelectrical constants of said transformer being such that the square ofthe surge impedance of the transformer equals the product of the surgeimpedance of the last mentioned transmission line and the resultantimpedance of the radiating elements.

ERNEST J. STERBA.

